if i needed to move one dropout/chainstay/seatstay closer to the centerline of a frame while insuring i did not disturb the other side, i would support the frame on a large bench with the side of the rear triangle i did NOT want to alter, hanging over it and closest to the floor. so as the headtube, BB and seattube only touched the bench. then apply pressure to the side that i wanted to move inward.

OTOH, it's entirely possible that the side that needs to be permanently moved will have to be moved beyond the other side before any permanent change can be made. is so, you're in a pickle. progress could possibly be made by applying the force closer to the chainstay's short support bridge near the BB. and, god willing, not dent the chainstay in the process.

but before i went to any trouble at all with bending a frame i would take a LOT of time a trouble (applying a straight edge along the chainring and rear cog at least, not that i think the OP has not performed due diligence) to determine that it needed to be done, although, i'll admit that 118mm width measurement is compelling...